This invention relates to the retractor (take-up device) of a seat belt, and more particularly to an improvement of a retractor which is equipped with a clamping mechanism adapted to directly hold the webbing to minimize the amount of pullout of the webbing in an emergency.
In a conventional seat belt device for safely securing an occupant on the seat in the vehicle, the webbing is held wound a certain length on the winding shaft of the retractor even when the seat belt is used. Hence, even an emergency locking type retractor equipped with an emergency locking mechanism which physically locks the rotation of the winding shaft in the direction of pulling out the webbing (hereinafter referred to as "a webbing pull-out direction", when applicable) for instance with the aid of inertia sensing means which operates in response to abrupt acceleration, collision or deceleration suffers from a difficulty that the webbing is somewhat stretched in an emergency, even though the rotation of the winding shaft in the webbing pull-out direction has been locked. This is because, upon emergency, tension is applied to the webbing wound on the winding shaft to tighten it. If the webbing is stretched in this manner, it impairs the performance of holding an occupant on the seat. Thus, it is necessary to minimize the amount of pull-out of the webbing from the retractor in an emergency.
In order to minimize the amount of pull-out of the webbing from the retractor in an emergency, there have been proposed a variety of retractors equipped with a clamping mechanism which is adapted to directly clamp the webbing at the outlet of the retractor.
For instance, clamping mechanisms disclosed by U.S. Pat. No. 5,127,598 and Japanese Patent Application (OPI) No. 112750/1991 (the term "OPI" as used herein means an "unexamined application") are designed as follows: The clamping mechanism comprises a clamp member which includes a clamp for clamping the webbing on the back of the base of the retractor; and a clamp holder for swingably holding the clamp. And a locking member, which is swung in association with the rotation of a latch member forming an emergency locking mechanism to engage with a ratchet wheel mounted on the winding shaft of the retractor, is supported by a transfer member. When the latch member rotates in the webbing pull-out direction together with the winding shaft, the transfer member is swingably turned by the torque of the winding shaft which is transferred through the locking member, so that the clamp member is swingably turned in a webbing clamping direction. Under this condition, the clamp member clamps the webbing on the back of the base of the retractor, to prevent the webbing from being pulled out of the retractor.
With the above-described clamping mechanism, the clamp tightens the webbing as follows: The latch member turned upon activation of the emergency locking mechanism, the latch member is turned to swing the locking member. After the locking member thus rotated is engaged with the ratchet wheel of the winding shaft, the transfer member swingably turned by the torque of the winding shaft swingably rotates the clamp holder, so that the clamp tightens the webbing.
As is apparent from the above description, in order to transfer the rotation of the latch member to activate the clamp, a number of transfer means are used. In each of the transfer means, energy loss due to its play cannot be neglected. Hence, the period of time is relatively long which lapses from the time instant that the inertia sensing means detects the collision until the clamp clamps the webbing. This delay in the clamp operation lowers the performance of holding an occupant on the seat. In addition, the clamping mechanism is large in the number of components, and intricate in construction, and accordingly high in manufacturing cost.